A mobile radio communication system, such as a UMTS (Universal Mobile Telecommunication System) type system, includes a mobile radio communication network communicating with mobile terminals or UEs (User Equipments) and with external networks. Traditionally, communications are facilitated using one or more radio base stations that provide radio coverage for one or more cell areas. Recent deployment of High Speed Downlink Packet Access (HSDPA) in operational 3G networks increases the downlink system capacity providing improved end-user experience by higher download speed and reduced round trip times. Enhanced Dedicated Channel (E-DCH) provides improved uplink performance in 3GPP Releases 6 and 7. New Medium Access Control layers (MAC-e/es) were introduced to support High Speed Uplink Packet Access (HSUPA) features like fast Hybrid Automatic Repeat Request (HARQ) with soft combining, reduced TTI length, and fast scheduling.
In spite of the fact that similar features have been introduced for HSDPA and HSUPA, there are several differences. In HSDPA, the High Speed Downlink Shared Channel (HS-DSCH) is shared in time domain among all users, but for HSUPA, the E-DCH is dedicated to a user. For HSDPA, the transmission power is kept more or less fixed and rate adaptation is used. However, this is not possible for HSUPA since the uplink is non-orthogonal, and therefore, fast power control is needed for fast link adaption. Soft handover is not supported by HSDPA, while for HSUPA soft handover is used to decrease the interference from neighboring cells and to have macro diversity gain.
The transport network links between base stations and radio network controller (Iub interface) and between radio network controllers (Iur interface) can be a bottleneck in the radio access network for HSUPA because the increased air interface (Uu) capacity does not always come with similarly increased transport network capacity. The cost of transport network links is high and may not decrease significantly. Given that congestion over a transport link cannot be solved by Transmission Control Protocol (TCP) efficiently because of lower layer retransmissions, HSUPA flow control can be used. The goal of Enhanced Uplink flow control is to avoid congestion on the Iub interface between the RNC and Node-B, i.e., to avoid congestion in the transport network (TN). For this purpose, a new control frame and a new Information Element for the E-DCH Iub/Iur data frame are introduced in 3GPP TS 25.427 V6.6.0 (2006-03), “UTRAN Iub/Iur interface user plane protocol for DCH data streams.” The new control frame is a Transport network layer (TNL) Congestion Indication (TCI) control frame (CF) and the new information element is the TCI. Based on the received E-DCH data frame sequence, the RNC can detect TNL congestion and indicate different types of congestions to the Node-B using the TCI CF. Based on the received TCI, the Node-B takes action to resolve congestion.
In the uplink, the user equipment terminal (UE) can be connected to more than one cell in soft handover (SHO). One of the SHO cells is the serving cell, and the remaining SHO cells are non-serving cells. Ideally, the radio links with the serving cell have the best radio quality, and the radio link with each non-serving cell also has acceptable radio quality. Serving cell selection is based on the radio link quality in the downlink direction from radio network to UE. But in real world systems, the non-serving cell may have better radio link quality than the serving cell in the uplink direction from the UE to the radio network. Unfortunately, typical EUL flow control assumes that the serving cell is the best cell for both downlink and uplink radio communications with the UE. (For purposes of this application, the term “flow” is used and is associated with a physical layer communications link that carries packets between the RNC and the UE.) Fairness among the flows is guaranteed only when this best cell assumption is correct. Under this assumption and in a congestion condition in the transport network, typical EUL flow control has the non-serving cell drop or discard packets received from the UE to reduce the congestion. This is not a problem as long as the serving cell really is the best cell in the uplink direction because those same packets are usually properly received in the serving cell anyway, so the redundancy normally provided in the non-serving cell is not needed.
But a problem with this typical approach occurs when the non-serving cell becomes the best cell at least in the uplink direction but still remains the non-serving cell, e.g., because the current serving cell still remains the best cell in the downlink. If the transport network experiences congestion for the UE flow, e.g., because the available uplink radio bandwidth for the flow is larger than the available uplink bandwidth for the flow in the transport network), then flow control continues to reduce congestion in the transport network by dropping packets at the non-serving cell even though it is the best cell currently for the uplink communication from the UE.
Consider the following example situation. Assume a good radio quality that allows the UE to transmit data over the radio interface up to, e.g., 1 Mbps, but the available transport network bandwidth for this flow is, e.g., 500 kbps. In this example, the system is transport network limited because 1 Mbps>500 kbps. The EUL flow control then avoids transport network congestion non-serving cells by dropping packets. In this example, if the data arrives from UE to a Node-B with 1 Mbps, the EUL flow control drops at least half of the packets to avoid transport network congestion when the available transport network bandwidth is only 500 kbps.
This packet dropping flow control is not appropriate when the non-serving cell uplink flow might have better quality then that for a flow to the serving cell. The result in this packet dropping situation at the non-serving cell is the need for increased RLC retransmissions for the dropped packets, which ultimately results in poor end-user quality of service and unfair end-use of resources.